[1] N Bengal Univ, Dept Phys, Darjeeling 734430, India
[2] Inter Univ Ctr Astron & Astrophys, Pune 411007, Maharashtra, India
[3] Univ KwaZulu Natal, Astrophys & Cosmol Res Unit, Sch Math Sci, ZA-4041 Durban, South Africa
[4] Univ Zululand, Dept Math Sci, ZA-3886 Kwa Dlangezwa, South Africa