We report the discovery of a new double-mode RR Lyrae variable ( RRd ) in the field of our Galaxy : CU Comae .
CU Comae is the sixth such RRd identified to date and is the most metal-poor RRd ever detected .
Based on BVI CCD photometry spanning eleven years of observations , we find that CU Comae has periods P _ { 0 } =0 . ^ { d } 5441641 ( \pm 0.0000049 ) and P _ { 1 } =0 . ^ { d } 4057605 ( \pm 0.0000018 ) .
The amplitude of the primary ( first-overtone ) period of CU Comae is about twice 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 _ { \odot } is estimated for CU Comae using an updated theoretical calibration of the Petersen diagram .
High resolution spectroscopy ( R=30,000 ) covering one full pulsation cycle of CU Comae was obtained with the 2.7 m telescope of the Mc Donald Observatory , and has been used to build up the radial velocity curve of the variable .
Abundance analysis done on the four spectra taken near minimum light ( 0.54 < \Phi < 0.71 ) confirms the metal poor nature of CU Comae , for which we derive [ Fe/H ] = -2.38 \pm 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 .