COMMISSION 27 OF THE I.A.U.
                   INFORMATION BULLETIN ON VARIABLE STARS
                               Number 2901

                                                         Konkoly Observatory
                                                         Budapest
                                                         3 June 1986

                                                         HU ISSN 0374 - 0676


        OPTICAL PHOTOMETRY OF A0538-66 DURING QUIESCENCE *1

The periodic recurrent transient source A0538-66, which undergoes X-ray
and optical outbursts every 16.6 days when it is in an active state [1-4]
also shows extended periods of inactivity [5]. It has been in such an off-
state since the end of 1982 [6].
   We report here on photometric observations of this source which have
been made since 1983, to detect the onset of a new cycle of activity, and
to study its off-state behaviour, e.g., to detect possible orbital light
variations.
   The observations were made with the five-colour Walraven photometer
[7] on the 90-cm telescope at ESO. Each observation consisted of 4 to 16
integrations of 32 seconds, preceded and followed by 4 such integrations
of the sky background (taken ~15" east of A0538-66). A diaphragm of 11.6"
diameter was used, which excluded contamination of the signal by a nearby
13.4 mag. star, located 13" south of A0538-66 (star R on the finding chart
given in [3]). All measurements were taken relative to the Walraven
standard star HD 39844; corrections for differential atmospheric
extinction were applied. A total of 228 observations on 147 nights were
made between 1983 (JD 2445656) and 1985 (JD 2446293). From the spread of
the results of individual observations obtained during a single night we
estimate the r.m.s. accuracy of a single observation to be +-0.09 mag in V,
+-0.03 mag in V-B, and +-0.05 mag in B-U.
   An overview of the quiescent behaviour of A0538-66 is shown in
Fig. 1, in which the long-term histories of the V magnitude (transformed
to the Johnson system using the transformation formula (eq. 4) given in
[8]), and of the Walraven colours V-B and B-U are displayed. In this
Figure we have also included previously discussed off-state data [5].
   It appears that the source remained near a quiescent magnitude V
~14.8, with small irregular brightness variations (with a total range of
~0.1 mag) occurring on a time scale of tens of days. Also on longer time
scales the visual brightness (averaged over time intervals of ~200 days)
shows a small secular increase, from V=14.85 +- 0.01 (m.e.) near JD
2445300 to V = 14.77 +- 0.01 near JD 2446300.


                          [FIGURE 1]

   Figure 1: Long-term quiescent behaviour of A0538-66. The upper diagram
   shows the V-Johnson magnitude averaged over time intervals of 
   approx. 20 days. The middle and lower diagram show the corresponding 
   behaviour in the Walraven colours V_B and B-U respectively.


   The off-state colours of A0538-66 show moderate variations over time
scales of tens of days with r.m.s. deviations of the average colours of
0.015 mag in V-B, and 0.03 mag in B-U; except during the beginning of the
off state (between JD 5225250 and 5225400 [6]) no significant correlation
is present in the colour-magnitude, and colour-colour variations.


                          [FIGURE 2]

   Figure 2: Periodogram of A0538-66 in the neighbourhood of the orbital
   period of the binary, 16.6515 days [4]. Residuals of the V magnitude,
   with respect to the long-term trend, are folded. Shown in the diagram
   is chi-square of the folded data with respect to the standard deviation
   of the unfolded data.


   In order to look for the possible presence of periodic variations of
the brightness of A0538-66 at the 16.6 day orbital period we have made an
epoch-folding period search in the residuals of the V, B, and U magnitudes
with respect to the smoothed long-term trend, mentioned above. (In case of
observations performed during an isolated period of about one month we
have simply subtracted the average magnitude during that month).
   The resulting periodogram for the V-band data (Fig. 2) shows that
there is no evidence in our data for a modulation of the brightness of
A0538-66 (at the 16.6 day outburst period) during its off state. By adding
artificial sinusoidal signals, with various amplitudes between 0.01 and
0.10 mag., to the data we estimate that any periodic signal in the V-band
has an amplitude less than ~0.02 mag. Similar upper limits have been
obtained for the B and U passbands.


             S. VAN AMERONGEN, F. TESSELAAR, J. VAN PARADIJS

                Astronomical Institute "Anton Pannekoek"
                       University of Amsterdam
          Roetersstraat 15, 1018 WB Amsterdam, The Netherlands


*1 Based on observations made at the European Southern Observatory



References:

1. White, N.E. & Carpenter, G.E. 1978, Mon. Not. R. Astr. Soc. 183, 11P. [BIBCODE 1978MNRAS.183P..11W ]
2. Johnston, M.D. et al. 1979, Ap.J. 230, L11. [BIBCODE 1979ApJ...230L..11J ]
3. Johnston, M.D., Griffiths, R.E. & Ward, M.J. 1980, Nature 285, 26. [BIBCODE 1980Natur.285...26J ]
4. Skinner, G.K. 1981, Space Sci. Rev. 30, 441. [BIBCODE 1981SSRv...30..441S ]
5. Pakull, M. & Parmar, A., 1981, Astron. Astrophys. 102, L1. [BIBCODE 1981A&A...102L...1P ]
6. Van Paradijs, J. et al. 1984, Mon. Not. R. Astr. Soc. 210, 863. [BIBCODE 1984MNRAS.210..863V ]
7. Lub, J. & Pel, J.W. 1977, Astron. Astrophys. 54, 137. [BIBCODE 1977A&A....54..137L ]
8. Van Paradijs, J. et al. 1985, Astron. Astrophys. Suppl. 63, 71. [BIBCODE 1986A&AS...63...71V ]