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The stars of the 'Real Housewives of New York City' say they feel bad Luann de Lesseps is missing their reunion taping for rehab ... but they're staying positive.

We got a few 'RHONY' cast members heading into Cipriani in NYC Tuesday to film their season 10 reunion episode, and our photog asked about Luann's absence.

Ramona Singer says all the housewives feel bad, but adds they'll be okay without her. Tinsley Mortimer offered Luann well-wishes and Sonja Morgan says Luann will be missed.

As we reported ... the countess had been spiraling out of control weeks before announcing she was entering rehab for a second time in less than a year.