(Updates with Tropical Storm Cristobal strengthening)

By Michael Christie

MIAMI, July 19 (Reuters) - The third tropical storm of the 2008 Atlantic hurricane season, Cristobal, gained strength just off the U.S. East Coast on Saturday and gale-force winds and heavy rains were expected to lash the Carolinas as the storm grazed the shoreline on a northeasterly path.

Hurricane Bertha, meanwhile, defied cool Atlantic waters and remained a hurricane while a strong tropical wave south of Jamaica was expected to develop into a depression -- the precursor to a storm -- as it headed toward Mexico's Yucatan Peninsula and the oil rigs of the Gulf of Mexico beyond.

Cristobal strengthened despite being over only marginally warm waters but was not seen becoming a hurricane, which requires winds of at least 74 miles per hour (119 km per hour), the U.S. National Hurricane Center said.

It was about 125 miles (195 km) east of Charleston, South Carolina, by 5 p.m. EDT (2100 GMT) and moving to the northeast parallel to the coastline at 7 mph (11 kph) with 45 mph (72 kph) winds, the Miami-based hurricane center said.

A tropical storm warning was in effect from South Santee River in South Carolina to the North Carolina/Virginia border and rainfall of up to 5 inches (13 cm) could be expected in coastal areas, the center said.

Bertha, far to the east in the open Atlantic, continued to display an ominous resilience as it raced over chilly waters in the direction of distant Iceland.

The first hurricane of the season -- now on its way to becoming one of the longest-lived Atlantic storms on record -- Bertha formed on July 3 near the Cape Verde islands off the coast of Africa, signaling an early start to what might turn out to be an active six-month hurricane season.