CU Comae: A new field double-mode RR Lyrae variable the most metal-poor discovered to date

Fire report the discovery of a new double-mode RR Lyrae variable (RRd) in t he 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 phot ometry spanning II years of observations, we find that CU Com has periods P -0 = 0.5441641 days (+/-0.0000049) and P-1 = 0.4057605 days (+/-0.0000018). The amplitude of the primary (first overtone) period of CU Com is about tw ice the amplitude of the secondary (fundamental) period. The combination of the fundamental period of pulsation P-0 and the period ratio of P-1/P-0 = 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 diagra m. High-resolution spectroscopy (R = 30,000) covering the full pulsation cy cle of CU Com was obtained with the 2.7 m telescope of the McDonald Observa tory and has been used to build up the radial velocity curve of the variabl e. An abundance analysis performed on the four spectra taken near minimum l ight (0.54 < phi < 0.71) confirms the metal-poor nature of CU Com, for whic h 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 var iables in our Galaxy.