The essential UML logic diagrams are the class diagrams: they represent the classes of objects that make up a program. Object diagrams are used as the basis to show scenarios of dynamic evolution of the software. The use of object diagrams to depict static structure is very rare. The purpose of this study is to investigate whether the use of static object diagrams can improve the comprehension of software systems. We conducted a study with 17 graduate students during a software engineering course. The students were asked to answer questions about a software system. The system was described either with a class diagram or with both a class diagram and an object diagram. The student asked multiple choice questions on four different systems. This study revealed that there is a statistically significant difference in the comprehension achievement for two of the systems. The effect of the presence of object diagrams can be classified of medium size. These results allow us to formulate new research questions that guide our future work in this area.