On the variability of four yellow supergiants―possible protoplanetary objectsARKHIPOVA, V. P; IKONNIKOVA, N. P; NOSKOVA, R. I et al.Pis′ma v Astronomičeskij žurnal. 1993, Vol 19, Num 5, pp 436-442, issn 0320-0108Article

The high-latitude F supergiant IRAS 18095+2704: a proto-planetary nebulaHRIVNAK, B. J; SUN KWOK; VOLK, K. M et al.The Astrophysical journal. 1988, Vol 331, Num 2, pp 832-837, issn 0004-637X, 1Article

The molecular emission of young preplanetary nebulaeBUJARRABAL, V; ALCOLEA, J; PLANESAS, P et al.Astronomy and astrophysics (Berlin. Print). 1992, Vol 257, Num 2, pp 701-714, issn 0004-6361Article