The Infrared Astronomical Satellite (IRAS) Point Source Catalog (PSC) contains a large number of point sources that emit strongly in the range, but have no counterparts in other astronomical catalogues. These are generally referred to as `unidentified' or unassociated sources. All these sources appear to have circumstellar envelopes (CSE) of gas and dust. We have chosen thirty-three such `unidentified' sources to study their evolutionary characteristics. The sources selected meet the following criteria: i) are still `unidentified', ii) are at a galactic latitude (to minimise the problems arising from source confusion), iii) have good quality flux densities (Quality Factor = 3) in the first three bands of the IRAS Survey, iv) have a flux density higher than 1 Jy at , and v) are in the declination zone between and to ensure ease of observation from Kavalur, India (longitude and latitude ).
Hu et al. (1993) found several post-AGB stars using the IRAS colour criteria. Habing et al. (1987) pointed out that non-variable OH/IR stars, located to the red side of the region occupied by the AGB stars in the IRAS colour-colour diagram, are proto-planetary nebula (PPN) candidates. In recent years evidence has grown that a number of high latitude supergiants with circumstellar dust shells and far-IRAS colours similar to planetary nebulae may in fact be post-AGB stars (e.g. Parthasarathy & Pottasch 1986). Quite a number of unidentified IRAS sources with IRAS colours similar to planetary nebulae have been found to be PPN candidates ( Hrivnak et al. 1988, 1989; Likkel et al. 1987, 1991; van der Veen et al. 1989; Manchado et al. 1989; Garcia-Lario et al. 1990).
There are approximately 2300 IRAS sources in the PSC that meet the selection criteria described above. In this paper we included the sources for which we could get photometric observations in B,V,R,I bands. The number of photometric nights at Vainu Bappu Observatory, Kavalur are very few. We could carry out photometric observations of about 33 sources within the allotted observing time. We will continue to study the remaining sources.
In this paper we report an analysis of IRAS and optical photometric data of 33 unidentified IRAS sources. The sources were selected based on the above mentioned criteria. Out of the 33 sources studied by us, 30 have optical counterparts. The BVRI CCD photometric observations are presented for most of these stars.