2 The observational programme

2.1 The sample

All Northern and accessible Southern symbiotics brighter than $K\sim$ 8 and not fainter than 13 in V were chosen from the list of Allen (1984) and Kenyon (1986) as potential targets for observations. We reserved those in very crowded fields for observations at a later stage even though they fulfilled the above criteria. A few suspect or possible symbiotics (such as SS Lep) and those which were regular targets of AAVSO and VSNET observers (e.g.: NQ Gem, T Lyn) were also selected to be observed. Observations were scheduled at regular intervals in order to get good phase coverage, but bad weather often precluded a more complete set of data.

2.2 Observations

The observations were carried out using a LN₂-cooled InSb photometer at the Cassegrain focus of the 1.2 m telescope at PRL's Mt.Abu Infrared Observatory (72$\hbox{$^\circ$}$ 46$\hbox{$^\prime$}$ 45$\hbox{$.\!\!^{\prime\prime}$}9$ E, 24$\hbox{$^\circ$}$ 39$\hbox{$^\prime$}$ 10$\hbox{$.\!\!^{\prime\prime}$}9$ N; 1680 m) during 1995-97. A 2 mm aperture, corresponding to 26$\hbox{$^{\prime\prime}$}$ on the sky, was generally used, the chopper throw was $\sim$ 30$\hbox{$^{\prime\prime}$}$ and the chopping frequency was kept at 15 Hz for dewar 2 and 8 Hz for dewar 1. Apertures of 1 and 3 mm were used on certain occasions and the chopper throw was correspondingly adjusted. The details of our experimental system are given in Ashok et al. (1994). Two different dewars - Dewar 1 (CVF) and Dewar 2 (New) - were used for obtaining the data. The filter responses of these two systems at the operating temperature of 77 K are given in Fig. 1. Our experience with both the dewars shows that they are essentially identical. Hence, all the measurements are referred to in the same system. The PRL filters, particularly K and H, are quite close to the new ESO filters; hence photometric standard stars were selected from Bouchet et al. (1991) and their magnitudes in our system were adopted to be the same as their ESO magnitudes. This procedure is necessitated by the fact that we are still in the process of properly calibrating our own photometric system. All the programme stars had their own nearby "prime" comparison star from the above list with respect to which the star magnitude was calculated. On days where other standards were observed, intercalibration among them was done. These observations are accurate to $\pm 0.05$ mag.

  

Figure 1: Filter responses of the two dewars used for observations. They are traced from manufacturer's curves for the operating temperature of 77 K

2.3 Results

The data obtained during the course of this survey is presented in Table 1. It lists data for a total of 30 known or suspected symbiotics and related objects. Most of them are S-type symbiotics. The programme objects appear in order of increasing RA. Remarks are noted in the last column, where the first digit denotes the dewar used and the second one refers to the aperture in mm.

  

Table 1: Symbiotic stars and related objects: JHK observations. The two digits in the last column denote the dewar used (D) and the aperture in mm (A) respectively

 

Table 1: continued