Sirius Satellite Radio ( SIRI) and merger mate XM Satellite Radio ( XMSR) saw shares tumble Monday after both outlined plans to raise more capital.

The news came as Sirius announced preliminary results for its second quarter early Monday, saying it anticipates a 25% jump in both revenue and subscriber numbers from a year ago.

Sirius said it will sell about $375 million in common stock in a fixed-price public offering, with up to another $65 million more to come later concurrently with a private offering by XM. The exact number of shares Sirius will offer depends on the terms of the XM offering. XM said it plans to offer $550 million of senior notes, due in 2014, which will be exchangeable into shares of Sirius common stock.

Shares of Sirius dropped 37 cents, or 16.4%, to $1.88. XM slid $1.11, or 11.9%, to $8.17.

Earlier, Sirius said that revenue for the second quarter 2008 should be approximately $283 million, up from $226 million in the same period in 2007. The Thomson Reuters average estimate currently stands at $283.9 million. Sirius also said that it will have an adjusted loss from operations of $24 million, narrowing from an adjusted loss of $79 million a year ago. The company did not provide data on a per-share basis.

Additionally, Sirius said its subscriber count rose 25% from a year earlier to 8.92 million. Retail subscribers were up 7% in the second quarter, while OEM subscribers, which include automobile installations, surged 53%. Monthly churn, a measure of how many subscribers left the service, decreased to 1.6% from 2.1% in the year-ago period, and the conversion rate is estimated to increase to 48% from 47% last year.

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