The slender 46-story residential tower nearing completion on East 55th Street is actually two buildings in one, a dormitory for Marymount Manhattan College topped by 40 upscale rental apartments.

The building is structured as two separate units, each with its own entrance, elevators, address, and owner. The top 15 floors, called the Capri, are home to the one- to three-bedroom apartments, for which leasing begins Tuesday. The entrance to the Capri will be at 235 East 55th Street and adorned by a canopy. The owner is Arun Bhatia of Manhattan, who also developed the high-rise and owned the seven-story building that formerly stood on the site, between Second and Third Avenues.

The college, on East 71st Street with an enrollment of nearly 2,500, owns the building's lower 31 floors. Its dormitory entrance is at 231 East 55th Street. The dormitory floors are filled with 112 suites, each averaging 700 square feet with two bedrooms and kitchen and bathroom facilities and accommodating four to six students. A total of 493 students will live in Marymount's section of the building, which will also have two apartments for resident directors.

The project will be the first dormitory for the 66-year-old school and will bring its residential students - now scattered in four locations, including Brooklyn Heights - under one roof, said Paul Ciraulo, vice president of administration and finance at Marymount and the project's manager. The school had 518 such students in the academic year that just ended, he said. Mr. Ciraulo added that if more than 493 students want housing in the fall, the school will try to lease space near the dormitory.

The owners shared in the $57 million cost to the build the tower, with Mr. Bhatia securing private financing for his $18 million portion of the project. The college's $39.5 million share is being financed from the sale of tax-exempt bonds by the Dormitory Authority of the State of New York. The college will repay the bonds over 30 years.

The building which sets back at the 9th, 21st, 32nd, and 46th floors, has been designed by Costas Kondylis of Manhattan with a façade that reflects its separate uses. At its base, the tower is mostly brick and limestone; it becomes brick and glass on the dormitory floors. Mr. Kondylis said the top of the building "is mostly glass like a lighthouse," because of the floor-to-ceiling windows that are dominant features of the apartments.

The Capri's 36 one- and two-bedroom units, with 700 and 1,150 square feet, will rent for $3,200 to $5,850 a month. There are also two 1,850-square-foot three-bedroom apartments that will rent for $10,500 a month. The monthly rent for the two duplex penthouses, with 1,700 and 2,200 square feet, will be $13,500 and $15,500, according to Citi Habitats, a Manhattan rental brokerage and the project's leasing agent.

Mr. Bhatia said he bought the East 55th Street property in 1998 from the Guggenheim Museum because of its location, close to Midtown offices and two blocks from Sutton Place. Zoning allowed a 25-story building on the site without any special permits, but he said he was able to increase the project to its current height by buying the development rights over an adjacent building.

Mr. Bhatia said he initially envisioned constructing a 150-unit rental apartment building. But, he said, the fall of 1998 brought the Asian financial crisis, causing lenders to become more conservative. Though he said he had a commitment from a bank for the rental building, the current project "appealed to me because the college pre-leased 75 percent of the building and brought its own financing, taking away much of the market risk." Mr. Ciraulo said Marymount had been considering such a move for several years because "as the school grew, it was getting more demand from students outside Manhattan."

The cost for a student to live in the dormitory will be $8,500 for the academic year, he said. The first students will move in this September.

For his part, Mr. Bhatia got what he described as the highest and best floors. "In Manhattan, after location, views, light and air are important," he said. And because the building is only about 50 feet wide, there are no more than three apartments per floor.

Andrew S. Heiberger, president of Citi Habitats, said given the project's location, he expected the apartments to appeal to professional couples who want to walk to work.