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In this paper we describe a few phenomena that may affect the linewidth and/or the line shape of Ramsey fringes observed in an alkali-metal vapor cell with buffer gas. Experiments are performed on a vapor-cell-microwave-cavity arrangement in which 87Rb atoms successively experience laser pumping preparation (through a D2 line), microwave interrogation with a couple of coherent pulses, and clock signal detection by means of laser absorption. We show that the linewidth of the Ramsey central fringe is influenced by the Rabi pattern produced by a single microwave pulse. Moreover, cycle memory and atomic density effects can modify the shape of the Ramsey central fringe from the common sinusoidal behavior. A simple three-level model well reproduces the experimental observations. [ABSTRACT FROM AUTHOR]