Five colour photometry of the RRd star V372 Serpentis

Aims. The first U-band and new photometry of the RRd star V372 Ser is presented to determine some parameters of the star.Methods. In April, May 2007, 2812 frames were obtained at Konkoly and Teide Observatories and 1508 V observations were collected from the literature. Fourier fitted light curves were derived in all bands.Results. The non-linearly coupled frequencies -1, -1, i.e. periods days, d, , amplitudes mag, mag, and phases have been found. has been found from averaging the amplitude ratio in the different bands i.e. the first overtone is the dominant pulsation mode. From the V observations, upper limits are given for secular change of the Fourier parameters. The period ratio and period put V372 Ser among the RRd stars of the globular clusters M3 and IC 4499; mass, luminosity, and metallicity estimates are given.