3 Discussion

The present photometry, in conjunction with available JHK data, can be used to study the variability of symbiotics. For example, variations due to orbital motion are very clearly seen in the three bands in case of UV Aur

(see Fig. 2). TX CVn also shows a phase-related modulation (see Fig. 3), especially in the K band. RW Hya shows a fair amount of scatter superposed on the small modulation (Fig. 4). The plot for AX Per (Fig. 5) hints at large intrinsic variability of the object. Data points in all the above figures are in different photometric systems; nevertheless, the observed scatter is more than can be accounted for by differences in the photometric systems.

  

Figure 2: Phase diagram for UV Aur. The orbital ephemeris is taken from Zakarov (1951). Filled symbols are our data and open symbols are data available in literature. J data are denoted by triangles, H by squares and K by pentagons

  

Figure 3: Same as above but for TX CVn. The orbital ephemeris is taken from Kenyon & Garcia (1989)

  

Figure 4: Same as above but for RW Hya. The orbital ephemeris is taken from Schild et al. (1996)

  

Figure 5: Same as above but for AX Per. The orbital ephemeris is taken from Skopal (1991)

Some of our data points have apparently anomalous values. We have checked our data carefully and definitely rule out instrument malfunctions or bad sky as the cause. It is interesting to note that such cases have been observed before. The symbiotic nova AS 338 was observed to be anomalously faint by Munari (1992). Munari et al. (1992) have found AS 327 to be unusually bright. The K magnitudes of AX Per as given by Swings & Allen (1972) and Szkody (1977) suggest strong variability. Kenyon & Gallagher (1983) found no evidence for the variability of AX Per, but our observations in the K band show an unusually faint data point. It seems that some symbiotics may occasionally show such flares or dips, the cause of which should be investigated in detail.

There still are significant gaps in the phase coverage of symbiotic stars in our sample and more complete light curves are therefore desirable. Pointed observations at specific phases are needed to confirm the reality of some of the unusual observations. This programme will be continued in the coming years also in order to study the various aspects outlined in Sect. 1. This data would be particularly useful in studying objects with little reported near-IR photometry - NQ Gem, ZZ CMi and T Lyn, for example.

Acknowledgements

Research work at PRL is supported by funds from Dept.of Space, Govt. of India. This research has made use of NASA's ADS Abstract Service. The authors thank Watson. P.V. and Anandmayee Tej for help with the observations.