Sirius XM Radio Inc. said Tuesday it returned to profit in the first quarter, as it added subscribers amid a recovery in auto sales and kept a tight rein on costs. But revenue came in slightly below analysts' average estimate, and shares fell 9 cents, or 7.3 percent, to $1.14 in morning trading. The stock has slipped as low as 30 cents a share over the past year on concerns that the recession would push Sirius into bankruptcy court. Business has picked up along with better auto sales, where Sirius sells many of its new subscriptions. The company added 171,441 net subscribers during the period to end the quarter with over 18.9 million subscribers. At the end of the year-ago period it had 18.6 million subscribers. Sirius posted earnings of $41.6 million, or a penny per share, compared with a loss of $238.8 million, or 7 cents per share, a year ago.