Photometric results are given in Tables 3-7. For the stars NSOri and XXOri the quality of the U band measurements was poor and corresponding measurements were thus omitted in the tables. Table 2 (click here) shows the ranges of variation observed for each star in each filter. Because of the limited number of observation points, especially in the B and U band, these ranges are likely to be only lower limits of the amplitudes of variation. In the field of YYOri and CEOri/AOOri two reference stars were present. One of them, NGC 1977-38 has known magnitude and spectral type and allowed us to translate the relative magnitudes of YYOri to absolute magnitudes. The reference star UBVM51765 is of unknown spectral type, but photometric measurements in the UBV bands were made and are used to estimate the UBV magnitudes of CEOri and AOOri. The star Parenago1943 who has known spectral type and BV magnitudes was used to estimate NSOri's magnitudes. Resulting observed magnitudes are given in Table 1 (click here). We applied the periodogram analysis (see Horne & Baliunas 1986) to the data in order to search for a periodic component. Periods were searched for within the time interval 2 - 10 days, except for CEOri for which a interval of 2 to 20 days was taken. We now briefly comment on individual objects.
| Filter | YYOri | CEOri | AOOri | NSOri |
| I | 12.09 | - | - | 12.41 |
| R | 12.90 | - | - | 14.01 |
| V | 13.82 | 15.29 | 15.42 | 15.27 |
| B | 14.74 | 15.98 | 16.37 | 16.81 |
| U | 14.69 | 16.00 | 16.73 | - |
| Filter | YY Ori | SYOri | CEOri | NSOri | XXOri | V481Ori | AOOri |
| I | 1.14 | 0.30 | 0.55 | 0.29 | 0.62(1.31) | 0.22 | 1.84 |
| R | 1.43 | 0.38 | 1.12 | 0.31 | 0.85 | 0.29 | 2.67 |
| V | 2.16 | 0.57 | 1.46 | 0.73 | 1.14 | 0.32 | 3.04 |
| B | 2.62 | 0.62 | 1.89 | (0.86) | (0.27) | 0.46 | 3.60 |
| U | 3.25 | 0.52 | 1.83 | - | - | (0.40) | 4.14 |
YYOri is an extremely active CTTS with spectral type K5e and a strong UV excess. It is the prototype of this peculiar class of CTTSs. In a previous publication we reported a quasi-periodic modulation of the star's light curve with a period of about 7.6 days and showed that this variation was due to the presence of a hot spot with temperature 6060K covering about 10% of the projected stellar disk area. The present set of observations does not provide any evidence for periodic or quasi-periodic behavior. The light curve shows an irregular variability on the time scale of 3 to 4 days.
A possible period of 5.7 days for the K3-star SYOri was reported by Gahm et al. (1993). They interpreted the variation as being due to the presence of a cold spot on the stellar surface. Applying the periodogram analysis to our data the largest peak for the I band is found at 8.2 days, but with a confidence level of less than 90%. The periodograms of the other filters have no significant peak with a confidence level much more then 50%. The light curve of the first half of our observation campaign shows two minima of different value, while the second half shows more irregular variations with a different shape. The amplitude of variation is less than 1mag in all filters although there are not enough measurements in the U band to accuratly determine the amplitude of the U variability. If it is well correlated with the variability in the other filters, one may expect the amplitude in U to exceed 1mag. The peculiar form of the light curve with two different values of the minima stands in contrast to simple spot models.
CEOri is a binary system (Hartigan et al. 1994) where the stars
are separated by 13.7'' and have spectral types M1 and M3.
Applying the
periodogram analysis, one peak is found at
days in the filters I, R and V
with a confidence level at about
99%. Also, a second equally important peak arises between 18 and 19
days. Since each observation intervall was about 10 days, it is impossible
to determine a reliable period.
The amplitude of variation is more than 1mag in filter R and V which
rules out that a cold spot causes the brightness modulation, since TTauri
stars with cool spots show typical amplitudes of variation of less than
1mag in filter V (see Herbst et al. 1994).
The maximum is quite flat, in contrast to CTTSs with spots which
show a more sinusoidal modulation. It is also interesting that the light
curve decreases slowly in the first half of our observation campaign while
it seems to drop off rather rapidly in the second half.
NSOri was identified as a flare star by Haro (1976) and has a spectral type of M1. The variability is quite irregular and there is only a weak correlation between the variations in different filters. However, the error is on the same order of magnitude as the variations.
XXOri is a flare star (Haro 1976) with spectral type M2.5. The magnitude variations of XXOri in the different filters are quite well correlated and there seems to be a quasi-periodic behaviour in the first half of the light curve. However, the periodogram analysis did not confirm the presence of a well-defined period in the data set.
V481Ori is of unknown spectral type but is likely to be a T Tauri
star. Duncan (1993) measured a
of 21km s-1 and found that it is probably not a binary
system. Furthermore the star was identified as a X-ray source by
Gagné et al. (1995).
The result of the periodogram analysis shows a maximum at a period of 4.89
days with a confidence level of at least 99% in all filters except U
where there are too few observations. A second equally important peak
arises at period of 4.2 days. Figure 1 (click here) displays the periodogram
in the filter I and Fig. 3 (click here) shows the light curve
with a
sinusoidal fit of 4.9 day period and Fig. 4 (click here) displays the
phased sinus curve fitted to the phased data points. The rms error are
typically of the order of 0.05.
Figure 1: Periodogram of V481Ori in filter I. The dashed horizontal
lines respresent the confidence levels
Figure 2: Plots of V-I and V-R color changes as a function of V magnitude
changes for all 5 YYOrionis stars
Figure 3: CCD photometric observations of V481Ori in UBVRI. The magnitude
values given here refer to the deviation from the average observed
magnitude. The solid lines shows the best-fit sine curves for a period
of 4.9 days
Figure 4: Phased folded photometric data for V481Ori. The solid lines
represent the best sinusoidal fit to the data in each filter
The spectral type is not known for AOOri. The light curves show important amplitudes in all filters: from less than 2 mag in the I band to more than 4 mag in the filter U. The dramatic changes in photometry may suggest that it is a CTTS. The periodogram shows no significant peak.
