425

Filed by Sirius Satellite Radio Inc.
Pursuant to Rule 425 under the
Securities Act of 1933 and deemed filed
pursuant to Rule 14a-6 under the
Securities Exchange Act of 1934

Subject Company: XM Satellite Radio Holdings Inc.
Commission File No.: 0-27441

This communication contains “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. Such statements include, but are not limited to, statements about the benefits of the business combination transaction involving Sirius Satellite Radio Inc. and XM Satellite Radio Holdings Inc., including potential synergies and cost savings and the timing thereof, future financial and operating results, the combined company’s plans, objectives, expectations and intentions with respect to future operations, products and services; and other statements identified by words such as “anticipate,” “believe,” “plan,” “estimate,” “expect,” “intend,” “will,” “should,” “may,” or words of similar meaning. Such forward-looking statements are based upon the current beliefs and expectations of SIRIUS’ and XM’s management and are inherently subject to significant business, economic and competitive uncertainties and contingencies, many of which are difficult to predict and generally beyond the control of SIRIUS and XM. Actual results may differ materially from the results anticipated in these forward-looking statements.

The following factors, among others, could cause actual results to differ materially from the anticipated results or other expectations expressed in the forward-looking statement: general business and economic conditions; the performance of financial markets and interest rates; the ability to obtain governmental approvals of the transaction on a timely basis; the failure of SIRIUS and XM stockholders to approve the transaction; the failure to realize synergies and cost-savings from the transaction or delay in realization thereof; the businesses of SIRIUS and XM may not be combined successfully, or such combination may take longer, be more difficult, time-consuming or costly to accomplish than expected; and operating costs and business disruption following the merger, including adverse effects on employee retention and on our business relationships with third parties, including manufacturers of radios, retailers, automakers and programming providers. Additional factors that could cause SIRIUS’ and XM’s results to differ materially from those described in the forward-looking statements can be found in SIRIUS’ and XM’s Annual Reports on Form 10-K for the year ended December 31, 2006, and Quarterly Reports on Form 10-Q for the quarters ended March 31, 2007 and June 30, 2007, which are filed with the Securities and Exchange Commission (the “SEC”) and available at the SEC’s Internet site (http://www.sec.gov). The information set forth herein speaks only as of the date hereof, and SIRIUS and XM disclaim any intention or obligation to update any forward looking statements as a result of developments occurring after the date of this communication.

Important Additional Information and Where to Find It

This communication is being made in respect of the proposed business combination involving SIRIUS and XM. In connection with the proposed transaction, SIRIUS has filed with the SEC a Registration Statement on Form S-4 containing a definitive Joint Proxy Statement/Prospectus and each of SIRIUS and XM may file with the SEC other documents regarding the proposed transaction. The Joint Proxy Statement/Prospectus was first mailed to stockholders of SIRIUS and XM on or about October 9, 2007. INVESTORS AND SECURITY HOLDERS OF SIRIUS AND XM ARE URGED TO READ THE JOINT PROXY

STATEMENT/PROSPECTUS, AS WELL AS OTHER DOCUMENTS FILED WITH THE SEC CAREFULLY IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION.

Investors and security holders can obtain free copies of the Registration Statement and the Joint Proxy Statement/Prospectus and other documents filed with the SEC by SIRIUS and XM through the web site maintained by the SEC at www.sec.gov. Free copies of the Registration Statement and the Joint Proxy Statement/Prospectus and other documents filed with the SEC can also be obtained by directing a request to Sirius Satellite Radio Inc., 1221 Avenue of the Americas, 36^th Floor, New York, NY 10020, Attention: Investor Relations or by directing a request to XM Satellite Radio Holdings Inc., 1500 Eckington Place, N.E. Washington, DC 20002, Attention: Investor Relations.

SIRIUS, XM and their respective directors and executive officers and other persons may be deemed to be participants in the solicitation of proxies in respect of the proposed transaction. Information regarding SIRIUS’ directors and executive officers is available in its Annual Report on Form 10-K for the year ended December 31, 2006, which was filed with the SEC on March 1, 2007, and its proxy statement for its 2007 annual meeting of stockholders, which was filed with the SEC on April 23, 2007, and information regarding XM’s directors and executive officers is available in XM’s Annual Report on Form 10-K, for the year ended December 31, 2006, which was filed with the SEC on March 1, 2007 and its proxy statement for its 2007 annual meeting of stockholders, which was filed with the SEC on April 17, 2007. Other information regarding the participants in the proxy solicitation and a description of their direct and indirect interests, by security holdings or otherwise, is contained in the Joint Proxy Statement/Prospectus filed with the SEC.

***

The following is a letter first sent to certain Sirius stockholders on October 23, 2007.

YOUR VOTE IS IMPORTANT!

October 22, 2007

Dear Stockholder:

Our records indicate your vote has not yet been received for the special meeting of stockholders of Sirius Satellite Radio Inc. to be held in The Auditorium at the Equitable Center, 787 Seventh Avenue, New York, New York 10019, on Tuesday, November 13, 2007 at 9:00 a.m., local time. Please take a moment right now to ensure that your shares are represented at this important meeting by signing, dating and returning the enclosed duplicate proxy card. Your vote is very important regardless of the number of shares that you own.

At the special meeting, stockholders will be asked to approve proposals in connection with the combination of SIRIUS and XM Satellite Radio Holdings Inc. The Agreement and Plan of Merger, dated as of February 19, 2007, by and among SIRIUS, Vernon Merger Corporation, a wholly-owned subsidiary of SIRIUS, and XM is more fully described in the proxy statement that we mailed to you on or about October 9, 2007.

The SIRIUS board of directors recommends that you vote FOR the proposal to amend SIRIUS’ certificate of incorporation to increase the number of authorized shares of common stock, FOR the proposal to approve the issuance of SIRIUS common stock and SIRIUS Series A convertible preferred stock in the merger and FOR the proposal to approve any motion to adjourn or postpone the special meeting to a later date or dates if necessary to solicit additional proxies.

In order to complete the merger with XM, the holders of a majority of the outstanding common shares of SIRIUS’ common stock must approve the proposals to amend SIRIUS’ certificate of incorporation and to issue SIRIUS common stock and SIRIUS Series A convertible preferred stock. Your failure to vote will have the same affect as a vote against these proposals and the combination of SIRIUS and XM.

As indicated on the proxy card, you may vote by internet or by signing, dating and mailing your proxy card in the postage-prepaid envelope provided.

Please vote today. Remember — every vote counts. If you have any questions or need a copy of the proxy statement, please call our proxy solicitor MacKenzie Partners, Inc. at (800) 322-2885 (toll-free) or (212) 929-5500 (collect).

Thank you for your cooperation and your continued support.

Sincerely,

Mel Karmazin

Chief Executive Officer

Important Additional Information and Where to Find It

This communication is being made in respect of the proposed business combination involving SIRIUS and XM. In connection with the proposed transaction, SIRIUS has filed with the SEC a Registration Statement on Form S-4 containing a definitive Joint Proxy Statement/Prospectus and each of SIRIUS and XM may file with the SEC other documents regarding the proposed transaction. The Joint Proxy Statement/Prospectus was first mailed to stockholders of SIRIUS and XM on or about October 9, 2007. INVESTORS AND SECURITY HOLDERS OF SIRIUS AND XM ARE URGED TO READ THE JOINT PROXY STATEMENT/PROSPECTUS, AS WELL AS OTHER DOCUMENTS FILED WITH THE SEC CAREFULLY IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION.

Investors and security holders can obtain free copies of the Registration Statement and the Joint Proxy Statement/Prospectus and other documents filed with the SEC by SIRIUS and XM through the web site maintained by the SEC at www.sec.gov. Free copies of the Registration Statement and the Joint Proxy Statement/Prospectus and other documents filed with the SEC can also be obtained by directing a request to Sirius Satellite Radio Inc., 1221 Avenue of the Americas, 36th Floor, New York, NY 10020, Attention: Investor Relations or by directing a request to XM Satellite Radio Holdings Inc., 1500 Eckington Place, N.E. Washington, DC 20002, Attention: Investor Relations.