High-precision timing of the millisecond pulsars PSR B1937+21 and PSR B1821-24 are conducted at 1.4 and 1.7 GHz with the high rate of observations of 10-15 times per month at Nançay. Time Of Arrival (TOA) and pulsar flux density are both measured. The rms of the daily post-fit TOA residuals are at the level of 0.5μs and 3.0μs, respectively. A characterization of refractive scintillation events produced by the Ionized Interstellar Medium (IISM) both in the TOA residuals and flux density variations is discussed.

Timing at Nan^ay is conducted with a pre-detection dedispersion system. The analysis of the TOAs was done with our software AnTiOPE solving for the classical pulsar parameters. The flux densities of the pulsars are averaged over ~30 or ~70 minutes. Calibration of the system temperature and efficiency of the telescope is good at the 10% level and diffrative scintillation limits this calibration to a comparable level (~35 scintels for PSR B1937+21 over 30 minutes and 7.5 MHz).