Tripadvisor (TRIP 16.74%) stock galloped ahead 18% through 11:05 a.m. ET Thursday after activist investor Starboard Value disclosed that it has taken a 9% stake in the travel advisor.

Calling the company "undervalued" (at the time it bought the shares -- we'll have to see if it remains undervalued now that it's up 17%), and "an attractive investment opportunity," Starboard plans to meet with management to discuss ways to improve the stock's price even further.

Although I have to say, a 17% one-day prop is already quite an improvement!

Simple green arrow going up.

Image source: Getty Images.

Momentum investing

Tripadvisor's an excellent prospect for the kind of stock that can be moved suddenly by a surprise headline. It was valued at less than $1.8 billion before today's announcement, so it only cost Starboard $160 million to build its 9% stake.

After the announcement, the stock has already gained more value than Starboard put into it.

Is Tripadvisor a buy?

Priced north of $2 billion today, Tripadvisor represents a potentially compelling value proposition. The company's debt load is modest -- only about $105 million. And while the stock costs nearly 38 times trailing earnings, analysts forecast Tripadvisor will also earn nearly $105 million next year, with even stronger free cash flow. Even valued just on the generally accepted accounting principles (GAAP) profit, the stock's forward P/E ratio is only about 20. And next year's earnings are expected to grow 40% compared to this year's.

Paying 20 times earnings for a 40% grower? Yeah, that sounds like a pretty "attractive investment opportunity" to me, too.