United Airlines reached an agreement to add 30 Embraer 175 regional jets to its United Express fleet, with 40 more on option, but did not say which partner will operate them or how many of the 50-seat jets they will replace.

United says only that it will remove “some” of the 50-seaters as the 30 76-seat Embraer 175s are brought into the fleet in 2014 and 2015, configured for 12 first class, 16 premium economy and 48 economy seats.

The 50-seaters operated by United Express carriers include both Embraer ERJ-145 and Bombardier CRJ200 aircraft, entailing 270 of the former and 75 of the latter as of Dec. 31, 2012, according to a Securities and Exchange Commission filing.

United owns and leases out 16 of the ERJ-145s, with an average age of about 13 years, according to the Aviation Week Intelligence Network fleets database.