CONSTRAINTS ON A SOFT-X-RAY EXCESS IN THE QUASAR 3C-279

We present the results of a three-week daily monitoring campaign of th e quasar 3C 279 by the X-ray satellites RXTE and ROSAT. A cross-correl ation provides no evidence for any time lag between the very similar s oft and hard X-ray light curves, and the source shows no significant s pectral, variability over the observing period. There is no evidence t o support the presence of a soft excess, with a 99 per cent upper limi t on any such component of 25 per cent of the total observed luminosit y in the 0.1-2 keV band (<3 x 10(38) W). This fraction (but not the lu minosity) is significantly lower than that of the soft excess observed in 3C 273.