  We study an anomaly in the X-ray flux (or luminosity) ratio between the OVII
21.6-22.1A triplet and the OVIII Lya line seen in classical T Tauri stars
(CTTS). This ratio is unusually high when compared with ratios for
main-sequence and non-accreting T Tauri stars (Telleschi et al. 2007). We
compare these samples to identify the source of the excess. A sample of
recently discovered X-ray stars with a soft component attributed to jet
emission is also considered. We discuss data obtained from the XMM-Newton
Extended Survey of the Taurus Molecular Cloud (XEST) project, complemented by
data from the published literature. We also present data from the CTTS RU Lup.
All CTTS in the sample show an anomalous OVII/OVIII flux ratio when compared
with WTTS or MS stars. The anomaly is due to an excess of cool, OVII emitting
material rather than a deficiency of hotter plasma. The excess plasma must
therefore have temperatures of <~2 MK. This soft excess does not correlate with
UV excesses of CTTS, but seems to be related with the stellar X-ray luminosity.
The spectra of the jet-driving TTS do not fit into this picture. The soft
excess depends both on the presence of accretion streams in CTTS and on
magnetic activity. The gas may be shock-heated near the surface, although it
may also be heated in the magnetospheric accretion funnels. The soft component
of the jet-driving sources is unlikely to be due to the same process.
