Second job for H1 - please help

I've just got a tenure-track assistant professor job with a 9-month appointment. I'm currently using H1B. The salary offered for this job is quite lower than a 12-month appointment. So I have to work for some extra money in the fourth quater of each year. Does anybody know whether it's legal to work for somebody else for 3 month while being employed full time by the university, and my h1 won't be affected? Or whom should I consult this if by any chance nobody knows the answer? Many many thanks for your kind help.