The term "YYOrionis star'' was introduced by Walker (1972) to define a subclass of classical TTauri stars (CTTSs). Whether this subclass really differs from the CTTS class is still an open question. A bona fide YYOrionis star displays inverse PCygni absorption components at the edge of the Balmer and CaII emission lines. These absorption components are red-displaced by typically 300 km s-1, which is a direct indication that matter accretes onto the star at close to free-fall velocity.
According to Walker's observations, the YYOrionis phenomenon seems restricted to CTTSs which have a strong UV excess in their spectral energy distribution (Walker 1983). Appenzeller (1977) then found that about 75% of the YYOrionis stars show a sizable UV excess while the same is true for only around 50% of all CTTSs. More recently, high-resolution spectroscopy of a large CTTS sample revealed that inverse PCygni profiles are present in most CTTSs in at least one of their Balmer lines, though they appear much shallower than in YY Orionis stars (Edwards et al. 1994). Thus the phenomenon of inverse PCygni profiles seems to be rather ubiquitous in TTauri stars, and one may doubt that YYOrionis characteristics are defining a peculiar class of young stellar objects. Nevertheless the question of why YY Orionis red-displaced absorption lines are much deeper than in CTTSs still remains.
Since the presence of inverse PCygni absorption components are a direct indication of accretion of matter onto the star, elucidating the difference between YYOrionis stars and "normal'' CTTSs may shed light upon the accretion process itself. Photometric observations in 1994 of class prototype YYOri (Bertout et al. 1996) revealed a quasi-periodic modulation of its light curve. It was also shown that this modulation was due to the presence of a large hot spot (or group of spots) on the stellar surface. The derived spot area covered about 10% of the stellar disk area, which is much larger than hot spots usually found in CTTSs. Comparing YYOri's characteristics to those of CTTSs shows that YYOri has a much more important accretion rate than usually determined for CTTSs (Hartigan et al. 1995), which suggested a link between the accretion spot size and the accretion rate. However, a comparaison of the spot's luminosities and accretion luminosities of a small sample of CTTSs (including YYOri) indicated that the spot's luminosity derived from the photometric variability is much smaller than the accretion luminosity of these stars. This and the fact that CTTSs with cold spots have similar accretion rates as CTTSs with hot spots led us to suggest that most of the accretion power might be dissipated in axially or spherically symmetric regions.
We recently observed a sample of YYOrionis stars (taken from Walkers 1972 original list) to study their photometric behaviour and compare them to "normal'' CTTSs. Section 2 describes the observations. In Sect. 3 we present and discuss the results, and we conclude with Sect. 4.