We report the discovery of a new double-mode RR Lyrae variable (RRd) in the field of our Galaxy: CU Comae. CU Com is the sixth such RRd identified to date and is the most metal-poor RRd ever detected. Based on BVI CCD photometry spanning 11 years of observations, we find that CU Com has periods P0 = 0.5441641 days (±0.0000049) and P1 = 0.4057605 days (±0.0000018). The amplitude of the primary (first overtone) period of CU Com is about twice the amplitude of the secondary (fundamental) period. The combination of the fundamental period of pulsation P0 and the period ratio of P1/P0 = 0.7457 places the variable on the metal-poor side of the Petersen diagram, in the region occupied by M68 and M15 RRd's. A mass of 0.83 M⊙ is estimated for CU Com using an updated theoretical calibration of the Petersen diagram. High-resolution spectroscopy (R = 30,000) covering the full pulsation cycle of CU Com was obtained with the 2.7 m telescope of the McDonald Observatory and has been used to build up the radial velocity curve of the variable. An abundance analysis performed on the four spectra taken near minimum light (0.54 0.71) confirms the metal-poor nature of CU Com, for which we derive [Fe/H] = -2.38 ± 0.20. This value places this new RRd at the extreme metal-poor edge of the metallicity distribution of the RR Lyrae variables in our Galaxy.
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