THE NEAR-INFRARED SPECTRUM OF U-SAGITTAE AT PRIMARY ECLIPSE

We have observed the Algol type binary U Sagittae before and during pr imary eclipse and near the first quadrature in the spectral region 830 0-8760 angstrom with the CARELEC spectrometer at the coude focus of th e 193 cm telescope of the Haute Provence Observatory. The purpose was to study the near IR spectrum of the secondary (a G4 giant) free from the contribution of the hot primary (a B8 main sequence star) and to c ompare it with those of some standard stars included between F5 and K3 as well as with the composite spectrum at phase 0.236. The comparison of the spectrum of the secondary with that of three standard stars su rprisingly indicates that it is very similar to that of the F5 standar d instead, as expected, to that of the G5 standard star. The disagreem ent between the spectral classification in the optical range and in th e near-infrared range is explained as due to the fact that the irradia tion from the primary is more effective in the IR, where the continuou s absorption coefficient of the secondary is maximum, than in the opti cal. In fact the heating in the IR probably affects just the upper lay ers, producing a stronger observable effect than in the optical, where the heating affects deeper layers.