1. Blommaert J.A.D.L.,van Langevelde H.J.,Habing H.J.,van der Veen W.E.C.J.,& Epchtein N.:1992, in Warner B., ed, ASP conf. ser. 30, Variable Stars and Galaxies, ASP,p.269
2. Catchpole R.M.:1990, in Bulges of Galaxies, ESO-CTIO Workshop, eds.,B.J.Jarvis & D.M.Terndrup,ESO p.111
3. Feast M.W.:1963,Mon. Not. R. astr. Soc.,125,367
4. Feast M.W.:1966,Mon. Not. R. astr. Soc.,132,495
5. Feast M.W.:1972, Vistas in Astronomy13,207