Context :

Aims : The first U -band and new BV ( RI ) _ { C } photometry of the RRd star V372 Ser is presented to determine some parameters of the star .

Methods : In April , May 2007 2812 U,B,V,R _ { C } ,I _ { C } frames were obtained at Konkoly and Teide Observatories , 1508 V observations were collected from the literature .
Fourier fitted light curves have been derived in all bands .

Results : The non-linearly coupled frequencies f _ { 0 } = ( 2.121840 \pm . 000001 ) ~ { } \mbox { cycles $ \cdot$ day$ { } ^ { -1 } $ } , f _ { 1 } = ( 2.851188 \pm . 000001 ) ~ { } \mbox { c $ \cdot$ d$ { } ^ { -1 } $ } , i.e .
periods P _ { 0 } = 0.4712891 \pm . 0000002 days , P _ { 1 } = 0.3507310 \pm . 0000001 d , P _ { 1 } / P _ { 0 } = 0.7441950 , amplitudes A _ { 0 } ^ { ( V ) } = 0.15399 mag , A _ { 1 } ^ { ( V ) } = 0.20591 mag , and phases have been found . A _ { 1 } / A _ { 0 } = 1.319 \pm . 008 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 .

Conclusions :