Shares of online travel-services provider Expedia Group Inc. were heading for their best day in two months on Thursday, as activist investor Starboard Value disclosed a large stake in its smaller rival, Tripadvisor Inc.

Activist investor Starboard Value disclosed a 9% stake in Tripadvisor on Thursday and said the stock was undervalued. Shares of Tripadvisor surged 17% after the hedge fund said it purchased the shares on the belief that they "represented an attractive investment opportunity.

Activist investor Starboard Value has built a 9% stake in Tripadvisor after the online travel-review company eschewed takeover offers in the past year.The detailsStarboard, the investment firm run by Jeff Smith,