设i为虚数单位，复数a+i1+i是纯虚数，则实数a等于______

∵a+i1+i=(a+i)(1?i)(1+i)(1?i)＝a+1+(1?a)i2＝a+12+1?a2i为纯虚数，∴a+12＝01?a2≠0，解得：a=-1．故答案为：-1．