1 Introduction

From an analysis of the IRAS point source catalogue data, several post-asymptotic giant branch (post-AGB) candidates were detected (Parthasarathy & Pottasch 1986; Pottasch & Parthasarathy 1988; Parthasarathy 1993a; Kwok 1993). The post-AGB stars, i.e. the nuclei of proto-planetary nebulae (PPNe) and planetary nebulae (PNe), were identified on the basis of far-infrared (IRAS) colours. In order to further understand the evolutionary status of the optical counterparts, it is often necessary to carry out multi-wavelength spectroscopy and photometry. The post-AGB stars detected from the IRAS data seem to form an evolutionary sequence (K, G, F, A to OB supergiant types) in the transition region from the tip of the AGB to the early stages of planetary nebulae (Parthasarathy 1993a,b, 1994; Parthasarathy et al. 1993). The PPNe and PNe have more or less the same far-IR colours (Parthasarathy & Pottasch 1986; Pottasch et al. 1988). Spectral classification based on low resolution optical spectra of several IRAS sources was carried out by Hrivnak et al. (1989). They have found several F and G type post-AGB candidates, but very few hot post-AGB candidates are known. In order to understand the evolution from cooler to hotter post-AGB types and then into the young PN stage, it is important to detect and study several hot post-AGB candidates. In order to detect hot post-AGB candidates and to determine their spectral characteristics, we have obtained low resolution blue spectra of several IRAS sources with far-IR colours similar to PNe and PPNe. In this paper we present the results based on spectra of selected IRAS sources obtained with the 1 m telescope of the Cerro Tololo Inter-American Observatory (CTIO), Chile.