What is the relationship between HorsePower and Electrical Frequency?

For example, the electrical motor is rated at 10Hp on a 60hz electrical supply. If the supply is dropped to 50hz or 40hz, what is the maximum horsepower available on that same motor? I cannot find a relationship between Hp and Hz or Torque and Hz. Can we assume the Torque (or Torque Load) stays the same? Thanks,