The Spin-Up Rate of the White Dwarf of GK Per

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.