1996A&AS..116..289Z

The paper presents the first evidence for long-term optical variability and colour behaviour of the X-ray-selected BL Lacertae object PKS 2155-304, which is the first example of its class. Our results show that the amplitude of the optical variations of PKS 2155-304 has been only about 1.4 magnitudes from its photometric history; no significant correlation between brightness and colours is found for PKS 2155-304. Our recent CCD photometry of PKS 2155-304 during September-October 1993 and July 1994 showed that it was the same brightness state as that in November 1991 (Smith et al. 1992).