V391 Peg: Identification of the two main pulsation modes from ULTRACAM u ' g ' r ' amplitudes

V391 Peg (HS 2201+2610) is an extreme horizontal branch subdwarf B (sdB) star, it is an hybrid pulsator showing p- and g-mode oscillations, and hosts a 3.2/sin i M-Jup planet at an orbital distance of about 1.7 AU. In order to improve the characterization of the star, we measured the pulsation amplitudes in the u'g'r' Sloan photometric bands using ULTRACAM at the William Herschel 4.2m telescope and we compared them with theoretical values. The preliminary results presented in this article conclusively show that the two main pulsation periods at 349.5 and 354.1 s are a radial and a dipole mode respectively. This is the first time that the degree index of multiple modes has been uniquely identified for an sdB star as faint as V391 Peg (B = 14.4), proving that multicolor photometry is definitely an efficient technique to constrain mode identification, provided that the data have a high enough quality. C() 2010 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim