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We report the discovery of the first recombination beta-line of hydrogen, H32beta, in MWC 349, using the JCMT. MWC 349, an early type emission line star, is believed to be surrounded by an edge-on Keplerian disk, where numerous, previously observed masing alpha-lines are generated. The double-peaked profile of H32beta, with a deep central dip (quite similar to the profiles of the masing alpha-lines) can not be interpreted exept in terms of exponential maser amplification in the disk. With its high frequency (367 GHz), this line should come from a deeper part of the disk than the corresponding alpha-lines which are more heavily extinguished by the free-free absorption. This discovery opens new possibilities for probing the kinematical and physical properties of the disk by comparing the observed parameters of the line with our numerical models of maser amplification in a Keplerian disk (e.g., Ponomarev, Smith, and Strelnitski, ApJ. April 1994). This investigation has been supported in part by the Smithsonian Institution Scientific Studies Program.
