May 1 (Reuters) - Activist investor Politan Capital Management on Monday nominated two director candidates to U.S. medical device maker Masimo Corp's five-person board, according to a regulatory filing.

The nominations ratchet up pressure on Masimo, which made headlines last year by adopting and later abandoning rules designed to deter activists from seeking board seats.

Politan, which owns a 9% stake in Masimo, proposed Quentin Koffey, its managing partner and chief investment officer, and former Johnson & Johnson executive Michelle Brennan as director candidates. Investors will vote at the annual meeting, scheduled for June 26, unless the two sides reach a settlement beforehand.

By asking shareholders to elect the pair, Politan is expected to push for greater board oversight and improved capital allocation one year after Masimo spent $1 billion to buy consumer technology company Sound United and its stock price tumbled on the news.

Masimo is currently valued at $10 billion.

A representative for Politan was not available for comment.

Masimo said on Tuesday that it had tried to work with Politan to find directors who were independent of the company and the hedge fund, but Politan had insisted Koffey would have to be one of the new board members.

Politan signaled interest in Masimo in August by saying in a regulatory filing that the company's stock price was too low.

Masimo defended itself with rules that would have forced Politan or other activists planning to nominate directors at the company to disclose the identities of the fund's limited partners and future plans to nominate candidates elsewhere.

The rules sent a chill through the activist industry as fund managers feared other companies could adopt similar defensive measures and force them to detail top-secret information, hurting their business.

Politan sued Masimo in Delaware court in October to reverse the rules, and Masimo abandoned them in February.

In March, Masimo announced governance changes including its intention to expand the board by two and to have all directors stand for election annually over the coming years.

As part of a shareholder settlement in 2015, Masimo committed to expanding the board to seven people.

Politan is pushing ahead with a portion of the lawsuit and has been joined by the California state teachers pension fund to try to stop Masimo's founder and chief executive, Joe Kiani, from receiving over half a billion dollars if there are changes to its governance, including its board of directors.

Politan has worked behind the scenes at Centene Corp , where new directors joined the board and the CEO stepped down. (Reporting by Svea Herbst-Bayliss; Editing by Josie Kao and Jamie Freed)