5)Beckwith, S. V. W. and Sargent, A. I.:The occurrence and properties of disks around young stars,pp.521-542.The University of Arizona Press,Tucson & London,1993.
6)Sargent,A.I.and Beckwith,S.V.W.:The detection and study of pre-planetary disks.Astrophysics and Space Science,212,pp.181-189(1994).