The reference string can be of several formats:
1: author [author ...] year [journal abbreviation] (eg Smith Jones 1999 ApJ)
2: author [author ...] et al year [journal abbreviation] (eg Smith et al 2001)
3: author [author ...] and author year [journal abbreviation] (eg Smith Miller and Jone 1998)
4: full reference string from a journal citation (eg Smith et al 2000, A&A 362, pp. 333-341)