2 IR observations and data calibration

The JHK photometry was obtained for the objects in Tables 1 and 2 with the 1.5 m Carlos Sánchez Telescope (CST) (Arribas & Martínez-Roger 1987) at the Observatorio del Teide (Tenerife), in June and October, 1994. Targets were chosen as in Paper I, with less emphasis on reproducing measurements in Probst (1983). A few WD targets were included. Their analysis will be presented elsewhere (Vennes et al.$\,$1998). GD274 was observed nightly, in October 1994, to check for reproducibility of results, and the distribution of the measurements are fully consistent with the photometric errors of each measurement.

A 15$^{\prime\prime}$ aperture was again employed with typical exposures of 2 - 4 minutes for each filter for single integrations. Averaged JH and K magnitudes are presented in Tables 3, 4 and 5 together with their corresponding 1 $\sigma$counting-statistic uncertainties. Typical signal-to-noise ratios are at least 5, and often higher. IR standard stars were observed nightly for calibration (Kidger et al.$\,$1992) and standard reductions were performed using the ESO Snopir software written by Olivia, Barbier, Schmider & Bouchet (P. Bouchet, private communication), available at the Instituto de Astrofísica de Canarias.

  

Table 3: Results from the June 1994 run at the CST. Each entry is the result of averaging of 3 or more integrations. When several entries are given, separate integration-series were performed. ^a after an object's name means weighted average of all its JHK measurements as given in Tables 3, 4 and 5

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The observations were performed in such a way that the $4^\prime \times 4^\prime$ area around each target, monitored in real time on a TV screen at the telescope's control, was carefully compared with a finder chart. Systematic checking of the accuracy of the coordinates was also carried out. As the CST has a high pointing accuracy, this task was in reality reduced to having confidence in the listed coordinates of the objects; we used the compilation by Kilkenny et al. (1988) in its latest corrected version (Heber, private communication) or original publications, aided by usage of SIMBAD, ADS or others. We have only found very few entry errors: only in one case was a possibility for misidentification encountered and the object has been dropped from this work. In no case was the 15$^{\prime\prime}$ photometer aperture contaminated by a field source (to a limiting V magnitude of about 15.5).

Our JHK values are plotted - February 1994, targets are not repeated - in Fig. 1, together with the corresponding broad energy distribution (see Sect. 3) for each object except for the CSPNs BD-13 842 and BD+33 2642. Whenever several observations exist, only the weighted average (as marked in Tables 3, 4 and 5) is plotted and further analyzed. The inverse of the square of the error on single measurements was used as weights in deriving the weighted averages. The error bar for the K values (and in a few cases also for the J or H ones) is large in the following cases and has not been plotted, for ease-of-display purposes: Feige 11, LSIV-14 11, PG 0242+132, PG 1338+481, PG 2102+037, PG 2111+023, PG 2128+146, PG 2158+082, PG 2218+022, PG 2345+241, PG 2352+181, PHL 678, TON788.

After our October observations were done, we were informed by the telescope staff that an occasionally wrongly positioned filter wheel might have affected part of our observations. This is indeed the case and can be clearly seen in Fig. 1 for all those stars where values in one of the JHK filters (mostly the H one) are very different (mostly lower) from the rest than it should be, given the values for the other two. This problem may then have affected at least the following stars: Feige 108, Feige 110, HZ1, HZ44, LSIV-14 11, PG 0122+214, PG 0133+114, PHL25, and PHL2726.

Of these stars only Feige 108 met the 2$\sigma$ JHK excess condition (see Sect. 4) and the rest were, therefore, excluded from further analysis. However, potential binary candidates may be hidden among them as seen from their excesses in two of the filters. Special attention should be paid in this regard to PG0122+214, PG0133+114, PHL25, and PHL2726.

In addition, from visual inspection of Fig. 1, it becomes apparent that several stars with large errors associated to their JHK photometry and/or data with unphysical appearance (i.e., which prevent them from fulfilling the 2$\sigma$ condition for their eventual excess), could still make good binary candidates - these being independent from and in addition to those displaying the filter wheel problem mentioned above. At least, that could be the case for BD+28 4211, LSIV+00 21, PG0215+183, PG 0242+132, PG 0856+121, PG 1409+605, PG 2102+037, PG 2111+023, PG 2158+082, PG 2159+051 and PG 2229+099. These stars have also not been analysed in the present paper but we suggest that they, together with the 4 objects mentioned earlier, be investigated in the future.

  

Table 4: Results from the October 1994 run at the CST. Notation is as in previous tables

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Table 5: Results from the October 1994 run at the CST (continuation)

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