SIMBAD references

In this paper, the post-1977 photometric observations of PKS 2155-304 are compiled and used to discuss the variation periodicity. Largest amplitude variations (ΔU=1.5mag; ΔB=1.65mag; ΔV=1.85mag ; ΔR=1.25mag; ΔI=1.14mag) and color indices ((B-V)=0.30±0.06; (U-B)=-0.72±0.08; (B-R)=0.62±0.07; (V-R)=0.32±0.04) are found. The Jurkevich's method and DCF (Discrete Correlation Function) method indicate possible periods of 4.16-years and 7.0-years in the V light curve.