Maintenance costs for owned aircraft

Suppose company A buys an aircraft and leases it to company B, then company B pays the maintenance costs, correct?

Now, suppose company A and company B both already operate three different kinds of aircraft. If company A buys a fourth aircraft type and leases to B,

then adding an additional aircraft type will affect maintenance costs.

So the question is, who is going to pay additional maintenance for the 4th aircraft type, A or B or both??

To me, it seems logical that company A should not pay extra, because it is not operating the aircraft, but maybe someone can confirm.

all aircrafts who you lease to other doesn’t count in your aircraft types

from the time when you offer the aircraft for leasing, it doesn’t count any more

Thanks, I see you replied before I finished editing :)