  We use hard X-ray light curves measured by the Chandra HETG and RXTE PCA
during the late rise and plateau phases of the 2002 March-April outburst of the
intermediate polar GK Per to determine that its X-ray pulse period P = 351.332
+/- 0.002 s. Combined with previous X-ray and optical measurements of the spin
period of the white dwarf, we find that its spin-up rate Pdot = 0.00027 +/-
0.00005 s/yr.
