Souyaku Toaru Majutsu no Index SS

is the title of two short stories included with artbooks CROSS and REBIRTH:
*Souyaku Toaru Majutsu no Index SS (CROSS): Short story included in artbook CROSS and focusing on Kuroyoru Umidori
*Souyaku Toaru Majutsu no Index SS (REBIRTH): Short story included in artbook REBIRTH and focusing on the Golden Dawn