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2 Observation

The observations of V 781 Tau in B and V bands were carried out on December 31, 1997 and on January 4, 1998 with the PI1024TKB CCD photometric system attached to the 100-cm reflector telescope at the Yunnan Observatory in China. The effective field of view of the photometric system is 6.5 square arc minute at the Cassegrain focus and its BV color system (Yang & Li 1999) approximates the standard Johnson BV photometric system. The three brighter stars except for the variable in the CCD field are used as the comparison and check stars, which are different from those used by other observers, so their coordinates together with those of the variable are given in Table 1. Integration time for each image is 100 seconds. In each of the two bands, 45 images during the primary eclipse and 38 images during the secondary eclipse have been obtained to determine times of minima of the system.
   
Table 1: The coordinates of the comparison and check stars

star
R.A. Dec.
  (1998.0) (1998.0)

V 781 Tau
$05^{{\rm h}}50^{{\rm m}}06^{{\rm s}}$ $ 26^\circ 57'41''$
comparison 1 05 50 15 26 59 47
comparison 2 05 50 18 26 56 42
check 05 50 13 26 56 34

   


The photometric program of the IRAF software is used to reduce the images. The results of the reductions show that the difference between the magnitudes of the check star and the comparison star 1 is constant within probable errors of $\pm{0.008}$ mag for the V band and $\pm{0.011}$ mag for the B band. However, the difference between the magnitudes of the check star and the comparison star 2 is $\pm{0.018}$ mag for the V band and $\pm{0.022}$ mag for the B band. So the star 1 is used as a comparison star to reduce our observations of V 781 Tau. Extinction corrections are not made because the comparison star 1 is very close to the variable.

From the observation made during the primary and the secondary eclipses, a mean epoch of the primary minimum light in the B and V bands and that of the secondary minimum light in the B and V bands are derived by means of the parabola fitting. The present times of minima together with those collected from the references are listed in Table 2, where the second column entry denotes the observation method: PG refers to the photographic observations and PHE to the photoelectric one.


   
Table 2: Times of minimum light for V 781 Tau

JD(hel.)
Meth. E (O-C)1 (O-C)2 Ref.
2400000+          

32881.460
PG -31812.5 -0.0193 0.001 Berthold (1983)
33950.515 PG -28713.0 -0.0110 0.005 ditto
34775.368 PG -26321.5 -0.0088 0.004 ditto
35540.371 PG -24103.5 -0.0149 -0.004 ditto
36610.285 PG -21001.5 -0.0098 -0.003 ditto
36957.442 PG -19995.0 -0.0041 0.002 ditto
38088.397 PG -16716.0 -0.0071 -0.003 ditto
38440.378 PG -15695.5 -0.0061 -0.003 ditto
39536.327 PG -12518.0 -0.0067 -0.006 ditto
40981.332 PG -8328.5 0.0003 -0.000 ditto
41329.345 PG -7319.5 -0.0002 -0.001 ditto
41330.381 PG -7316.5 0.0010 0.000 ditto
41337.279 PG -7296.5 0.0008 -0.000 ditto
42839.363 PG -2941.5 0.0044 0.003 ditto
43853.9088 PHE 0 -0.0008 -0.0022 Cereda et al. (1988)
43874.9482 PHE 61.0 -0.0009 -0.0023 ditto
44636.339 PHE 2268.5 0.0024 0.0014 ditto
44637.371 PHE 2271.5 -0.0003 -0.0013 ditto
46113.4117 PHE 6551. 0.0006 0.0010 Pohl et al. (1987)
46115.309 PHE 6556.5 0.0009 0.0013 ditto
46775.4651 PHE 8470.5 0.0004 0.0018 Cereda et al. (1988)
46788.3981 PHE 8508.0 -0.0007 0.0007 ditto
46798.4005 PHE 8537.0 -0.0006 0.0007 ditto
46802.3682 PHE 8548.5 0.0006 0.0020 ditto
46802.5396 PHE 8549.0 -0.0005 0.0009 ditto
48268.2285 PHE 12798.5 -0.0041 0.0000 Keskin et al. (1989)
48268.4014 PHE 12799.0 -0.0036 0.0005 ditto
48607.2728 PHE 13781.5 -0.0057 -0.0008 ditto
48607.4454 PHE 13782.0 -0.0056 -0.0007 ditto
50814.1688 PHE 20180.0 -0.0125 -0.0015 this paper
50818.1359 PHE 20191.5 -0.0119 -0.0008 this paper

         


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