The photospheric filling factor of the active binary II Pegasi

UBV and JHK photometry of the active singlelined binary II Peg, we performed in 1995, is presented. A method to determine the fraction of the photosphere covered by spots (filling factor) and to check the accuracy of generally assumed values of photospheric parameters has been developed. The procedure is based on the comparison between multiband fluxes and low resolution synthetic spectra weighted on the base of the spot filling factor and scaled with the ratio between the star radius and distance (R/d), so that we can also estimate the R/d ratio. A χ2 fit has been performed for II Peg observations close to the light maximum and minimum by assuming reliable values of the photospheric parameters. Although a unique solution cannot be reached, we found clear indication for a spot filling factor at light maximum ≥ 40%. We find that the same set of parameters that gives us the best fit solutions at light maximum also provides the best fit at light minimum. The resulting solutions are consistent with the observed amplitude of the photometric wave, and with the commonly accepted value of R, unspotted V magnitude and spectral classification for II Pegasi.