We use positron annihilation to study vacancy defects in GaAs grown at low temperatures (LT-GaAs). The vacancies in as-grown LT-GaAs can be identified to be Ga monovacancies, VGa, according to their positron lifetime and annihilation momentum distribution. The charge state of the vacancies is neutral. This is ascribed to the presence of positively charged As+ Ga antisite defects in vicinity to the vacancies. Theoretical calculations of the annihilation parameters show that this assignment is con... »