Digital books and study tools don't weigh down backpacks as heavy textbooks do. But fees for the codes to get them might be a financial burden for some college students, a new report found.

An analysis from the Student Public Interest Research Groups, state-based groups that advocate for causes like affordable textbooks, found that students in many courses might be asked to purchase online educational materials that require one-time digital access codes.

The codes are unique serial numbers that give students access to a variety of online materials, like digital books, study guides, homework assignments, quizzes and tests.

Sometimes students must purchase a textbook to obtain the necessary code, while in other cases the codes can be bought separately.

"They're the next frontier in the textbook affordability battle," said Nicole Allen, director of open education at the Scholarly Publishing and Academic Resources Coalition.

The average cost of a stand-alone access code, purchased at a campus bookstore, is about $100, the report found. The cost when bundled with a textbook varies depending on factors such as whether the textbook is digital or print, but averaged $126.

The analysis considered just 10 schools. But the mix of institutions studied — private, four-year colleges and public universities as well as community colleges — offered a snapshot of what students are probably encountering on campus these days, said Ethan Senack, higher education advocate at the Student Public Interest Research Groups.

Textbook costs have long been a sore spot for college students. According to a report in August from the Bureau of Labor Statistics, textbook prices have risen 88 percent over the past decade, significantly more than the increase of 63 percent for tuition and fees over the same period.

Students have used a variety of strategies to manage costs, Senack said, such as buying used textbooks, sharing a copy with another student, or renting physical or digital textbooks. More recently, a movement has emerged to promote "open source" textbooks, which are available free online.

The main reason students acquired an access code, the college store association's research arm said, was that their instructor required it.

Richard Hershman, vice president of government relations at the association, said pricing and distribution models for digital materials were evolving and student concerns should be taken into account. Faculty typically decide what materials are required, he said, and many instructors see online tools as helpful to students.

Student advocates said they worry that the proliferation of digital access codes might make it harder for students to use cost-cutting alternatives, such as sharing — or even skipping the textbook purchase entirely. Jeanne Ryder, a sophomore at Rutgers University, said she learned about the drawbacks of access codes last year, when she spent hundreds of dollars on a hardcover Italian textbook that was stolen, along with her backpack. The book had come with an online activation code, she said, but it was missing and the publisher told her she would have to buy a new one. She was unable to obtain a new replacement code, even though she had her receipt. She ended up borrowing a book from another student.

Ann Carrns writes for the New York Times.