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We develop a technique to measure radial velocities of stars from spectrathat present four sets of lines. The algorithm is an extension of thetwo-dimensional cross-correlation method TODCOR to four dimensions. It computesthe correlation of the observed spectrum against a combination of fourtemplates with all possible shifts, and allows also for the derivation of thelight ratios of the components. After testing the algorithm and demonstratingits ability to measure Doppler shifts accurately even under conditions of heavyline blending, we apply it to the case of the quadruple-lined system HD 110555.The primary and secondary components of this previously known visual binary(separation about 0.4 arcsec) are each shown to be double-lined spectroscopicbinaries with periods of 57 days and 76 days, respectively, making the system ahierarchical quadruple. The secondary in the 76-day subsystem contributes only2.5% to the total light, illustrating the ability of the method to measurevelocities of very faint components.Comment: Total of 15 pages including figures and tables, in emulateapj format.  To appear in The Astrophysical Journal, June 2007 issu

Cross‐Correlation in Four Dimensions: Application to the Quadruple‐lined Spectroscopic System HD 110555