Behaviour of the periods in double mode RR Lyrae, V26 in M15 M. Paparó, B. Szeidl, S. M. Saad, Z. Kolláth and M. S. Abu Elazm A&A, 391 2 (2002) 633-639 Published online: 02 August 2002 DOI: 10.1051/0004-6361:20020867